import sys
import math

def str_input():
    S = raw_input()
    if S[len(S)-1] == "\r":
        return S[:len(S)-1]
    return S

def float_to_str(num):
    return str("{:.10f}".format(num))

x1, y1, x2, y2 = map(float, str_input().split())

dx = x2 - x1
dy = y2 - y1
print float_to_str(math.sqrt(dx*dx + dy*dy)) import sys sys import math math def str_input():
    S = raw_input()
    if S[len(S)-1] == "\r":
        return S[:len(S)-1]
    return S str_input S = raw_input() S raw_input() raw_input if S[len(S)-1] == "\r":
        return S[:len(S)-1]
     S[len(S)-1] == "\r" S[len(S)-1] S len(S)-1 len(S) len S 1 "\r" return S[:len(S)-1] S[:len(S)-1] S :len(S)-1 len(S)-1 len(S) len S 1 return S S def float_to_str(num):
    return str("{:.10f}".format(num)) float_to_str return str("{:.10f}".format(num)) str("{:.10f}".format(num)) str "{:.10f}".format(num) "{:.10f}".format "{:.10f}" format num num num x1, y1, x2, y2 = map(float, str_input().split()) x1 map(float, str_input().split()) map float str_input().split() ().split () str_input split y1 x2 y2 dx = x2 - x1 dx x2 - x1 x2 x1 dy = y2 - y1 dy y2 - y1 y2 y1 print float_to_str(math.sqrt(dx*dx + dy*dy)) float_to_str math.sqrt(dx*dx + dy*dy) math.sqrt math sqrt dx*dx + dy*dy dx*dx dx dx dy*dy dy dy def float_to_str(num):
    return str("{:.10f}".format(num)) def float_to_str(num):
    return str("{:.10f}".format(num)) float_to_str y1, x2, y2 = map(float, str_input().split()) map(float, str_input().split()) y1 dx = x2 - x1 x2 - x1 dx x2, y2 = map(float, str_input().split()) map(float, str_input().split()) x2 dy = y2 - y1 y2 - y1 dy def str_input():
    S = raw_input()
    if S[len(S)-1] == "\r":
        return S[:len(S)-1]
    return S def str_input():
    S = raw_input()
    if S[len(S)-1] == "\r":
        return S[:len(S)-1]
    return S str_input x1, y1, x2, y2 = map(float, str_input().split()) map(float, str_input().split()) x1 y2 = map(float, str_input().split()) map(float, str_input().split()) y2
import sys
import math

def str_input():
    S = raw_input()
    if S[len(S)-1] == "\r":
        return S[:len(S)-1]
    return S

def float_to_str(num):
    return str("{:.10f}".format(num))

x1, y1, x2, y2 = map(float, str_input().split())

dx = x2 - x1
dy = y2 - y1
print float_to_str(math.sqrt(dx*dx + dy*dy))